San Diego Natural Museum


Cenozoic (Recent Life). Two periods: and

Quaternary Period: and Epochs

Time Geologic Development Life Forms

Holocene The Holocene may be an interval develops. Activities of Epoch between glacial incursions, typical of the mankind begin to affect world . The 10,000 Pleistocene Epoch and therefore not a separate of other species continues. ago to the epoch in itself. However, it is a period marked by the presence and influence of sapiens. During this , the retreat, sea levels rise, the warms, and form in some areas.

Pleistocene This epoch is best known as the "Great Ice ." The oldest species of Homo—— Epoch Ice sheets and other glaciers encroach and evolves. The and in the regions not 1.8 mill ion- retreat during four or five primary glacial periods. covered by ice are essentially the same as those 10,000 years At its peak, as much as 30% of the Earth's of the earlier Epoch. Mammalian ago surface is covered by glaciers, and parts of the evolution includes the development of large northern oceans are frozen. The movement of forms: woolly , woolly , the glaciers alters the landscape. , such as musk ox, , , , , the Great Lakes in , are formed as , and ground . ice sheets melt, and retreat. Global warming In the , large m ammals, such as begins after the , 18,000 , , , , saber- years ago. toothed cats, and ground , are entirely extinct by the end of this epoch.

Tertiary Period: Pliocene, , , , and Epochs

Time Geologic Development Life Forms

Pliocene The emergence of the Camels and horses are abundant throughout Epoch changes ocean circulation patterns and North America. Ground sloths also evolve and 5-1.8 m illion coincides with the formation of an ice cap. the Great American interchange between South years ago Plate tectonic interactions result in the uplift of and North America begins. the , formation of the Cascade continue to evolve , and the Range, and onset of strike-slip faulting on the —antecedents to Homo San Andreas Fault. In , the continue sapiens—develop late in the Pliocene in . to rise. In North America, and ordeodonts The glo bal climates become cooler and drier. become extinct.

Miocene Modern ocean currents are essentially forms are essentially modern, and Epoch established. A drop in sea level near the end of almost half of modern placental mammal families 24-5 m illion the Epoch isolates and dries up the are present. The ancestor of mastodons years ago , leaving evaporite deposits disperse into North America. on its floor. Almost all the modern groups of are The climate is generally cooler than the present, as well as the early seals and . Oligocene Epoch. A cold transantarctic ocean Many modern —herons, rails, ducks, current isolates the waters around , eagles, hawks, crows, sparrows—are present in and the becomes permanently frozen. Europe and . Higher primates u ndergo substantial evolution; advanced primates, including , are present in southern Europe and Asia. Carcharocles , the largest predaceous ever to have lived, inhabits the seas.

The coasts are submerged and develop. On land, replace forests over large areas on several .

Oligocene Tectonic plate movement is still very dynamic. Representatives of modern become Epoch Africa and Europe nearly collide, closing the the dominant life form, including 34-24 m illion Tethys Sea and leaving as a remnant the horses, pigs, true , rhinoceroses, years ago Mediterranean Sea. Volcanism and , and camels. Oreodonts diversify in fragmentation of western North America is North America. Early primates appear in North associated with the emplacement of major ore America, and early apes appear in . Many deposits. archaic mammals become extinct. The south eren ocean forms and the climate is The earliest representatives of modern generally temperate. Glaciation begins in cetaceans (baleen and "toothed" whales) evolve. Antarctica. Grasslands expand, and regions diminish.

Eocene Epoch and volcanic activity form the Early forms of , rhinoceros, , and 55-34 million Rockies in western North America. Erosion fills other modern groups such as evolve in years ago basins. Continental collisions between and Europe and North America. Creodonts and Asia culminate in the Alpine-Himalayan mountain ruminant evolve. . Antarctica and continue to Archaic whales (archeocete s) evolve from separate and drift apart. terrestrial meat-eating ungulates. Sirenians The climate is subtropica l and moist throughout ( and ) first evolve in the North America and Europe. shallow Tethys Sea.

Paleocene During the Paleocene, the vast inland seas of Mammalian life diversifies, spreading into all Epoch the Period dry up, exposing large major environments. Placental mammals 65-55 m illion land areas in North America and . eventually dominate the land, and many years ago Australia begins to separate from Antarctica, and differentiated forms evolve, including early splits from North America. A remnant ungulates (hoofed ), primates, , Tethys Sea persists in the equatorial region. and carnivores.

Mesozoic Era (Middle Life). Three periods: Cretaceous, , and .

Time Geologic Development Life Forms

Cretaceous The continents—while not in their current and other large peak as the Period positions on the Earth—are shaped much as dominant vertebrate life form on Earth. 144-65 million they are today. and Africa Dinosaurs extend their range throughout every years ago separate, and the ocean widens. A continent. Horned dinosaurs are common, while circum-equatorial sea, Tethys, forms between armored ankylosaurs and spiky nodosaurs are the continents of the Northern and Southern rare. Hemisphere. The westward movement of North In the shallow seas, live in great America forms the ancestral diversity. Ammonites are a dominant group. and the ancestral Sierra Nevada. Sea levels rise, Gastropods, , sea urchins flourish. submerging about 30% of the Earth's present The early flowering (angiosperms) , land surface. modern trees, and many modern types of The global cli mate is generally warm. The poles evolve. are free of ice. Near th e end of the Cretaceous Period, several mass occur, including the extinction of five major reptilian groups: dinosaurs, , , pleisosaurs, and . Extinctions also occur among ammonites, corals, and other marine invertebrates.

Jurassic The of Pangea begins to breakup Reptiles adapt to life in the sea, in the air, and on Period as North America separates from Eurasia and land. Dinosaurs are the dominant on land. 206-14 4 million Africa. The begins to form. Archaeopteryx, the first , evolves. years ago Tectonic plate along western North Early , extinct by the late T riassic, are America causes the Earth's crust to and succeeded by the first frogs, toads, and mountains form in the western part of the . continent. Mammals are small, shrew-like animals. forms are dominated by the and cycadeoides. and gingkoes are widespread.

Triassic covers nearly a quarter of the Earth's Life began to diversify after the end- Period surface. The Triassic Period, unlike the previous extinction. Early dinosaurs evolve. Many are 248-20 6 million periods, is marked by few significant geologic bipedal, fast, and relatively small. The largest years ago events. Toward the end of the Triassic Period, Triassic dinosaurs are only 20 feet (6 meters) in continental rifting begins to break apart the length—small when compared to later supercontinent. forms. The general clim ate is warm, becoming semiarid Marine reptiles evolve, such as ichthyosaurs and to arid. plesiosaurs. , s, ginkgoes, and conifers flourish. Mass extinctions occur at the end of the Triass ic Period, reducing some marine and terrestrial groups, such as the ammonites, therapsids, early reptiles, and primitive amphibians, by as much as 75 percent.

Paleozoic Era (Ancient Life). Six periods: Permian, , , , ,

Time Geologic Development Life Forms

Permian A single supercontinent, Pangaea, forms as is rich and diverse at the Period Earth's landmasses collide and merge. Pangaea beginning of the Permian period. Toward the end 290-24 8 million extends across all climatic zones and nearly of this period, mass extinctions occur among years ago from one pole to the other. This supercontinent is large groups of corals, bryozoans, , surrounded by an immense world ocean. and other invertebrates. 99% of all life perishes. Extensive glaciation persists in what is no w On land, insects evolve into their modern forms; India, Australia, and Antarctica. Hot, dry dragonflies and appear. conditions prevail elsewhere on Pangaea, and Amphibians decline in number, b ut reptiles deserts become widespread. undergo a spectacular evolutionary development of carnivorous and herbivorous, terrestrial and aquatic forms. Ferns and conifers persist in the cooler air.

Carboniferous Two major land masses form: (North The diversification of from the Devonian Period America, Greenland, Eurasia, and ) Period continues in both marine and freshwater 354-29 0 million to the north of the equator, and environments, though armored fish become years ago (South America, Africa, peninsular India, extinct. Benthic (bottom-dwelling) marine Australia, and Antarctica) to the south. Collisions communities include a variety of invertebrates: between Laurasia and Gondwana form major crinoids, blastoids, and . The mountain ranges. -forming sediments are ammonites are common in open marine waters. laid down in vast swamps. Insects, such as , flourish. The first

Global climatic changes occur, changing from reptiles evolve. warm and wet to cooler and drier. The result is a Land environme nts are dominated by plants, long interval of glaciation in the southern from small, shrubby growths to tall trees. Early hemisphere. club , horsetails, forest trees (Cordaites), and ferns are common.

Devonian Europe and North America collide, forming the This period is dominated by various forms of fish Period northern part of the ancestral Appalachian —armored fish, lungfish, and . 417-35 4 million . Europe and North America Ammonites evolve from an d become years ago straddle the equator. Africa and South America one of the dominant invertebrate forms. are positioned over the South Pole. As the ozone layer forms, the first air-br eathing The climate is generally warm and m oist. arthropods—spiders and —evolve on land. Amphibians evolve and venture onto land. Plant life, including lowland forests of giant psilophyta plants, develop and spread over the .

Silurian The North American, European, and Asian land Life in seas is still dominated by invertebrates: Period masses are situated on or near the equator. corals, arthropods, and crinoids. Rapid evolution 443-41 7 million and Baltica collide. Gondwana sits in occurs among suspension feeders, and pelagic years ago the south polar region. Shallow flooding of (open ocean) predators, such as nautiloids, continental areas deposits sediments; later become abundant. Fish evolve jaws. Late in the withdrawal of ocean water leaves oxidized "red Silurian Period, the first sharks appear. beds" and extensive salt deposits. The earliest land plants are represented by leafless, vascular plants called psilophytes.

Ordovician The barren continents of Laurentia, Baltica, Metazoan invertebrates are still the dominant Period , and Gondwana are separated by large form of life on Earth. Corals, crinoids, and clams 490-44 3 million oceans. Shallow seas cover much of North evolve, as well as the first early — years ago America at the beginning of the period. As the primitive fish with bony armor plates. seas recede, they leave a thick layer of Late in the Ordovician Period, mass e xtinctions . Later in the period, the seas recover of marine life occur, opening niches for benthic North America, depositing quartz, sandstones, (bottom-dwelling) and planktonic (floating, and more limestone. swimming) organisms.

Cambrian Sedimentary rocks (sandstone, shale, limestone, Marine metazoans with mineralized skeletons, Period conglomerate) form in shallow seas over the such as , bryozoans, corals, 540-49 0 million continents. Rodinia begins to break up into brachiopods, molluscs, arthropods, and years ago northern and southern portions. Gondwana in echinoderms, flourish. One group of arthropods, the south incorporates South America, Africa, the trilobites, are particularly dominant in the Antarctica, and Western Australia as well as shallow-water marine habitats. peninsular India and parts of Arabia. Plant life is limited to marine alg ae. The global climate is generally mild.

Precambrian Time. Three Eons: , , and .

Time Geologic Development Life Forms

Proterozoic The supercontinent Rodinia forms approximately Eukaryotes (single-celled organisms with a Eon 1.1 billion years ago. Plate tectonics slows to nucleus) evolve. These are more advanced 2.5 b illion approximately the same rate as the present. forms of algae and a wide variety of . years ago-540 Large mountain chains form as the continents Eukaryotes can reproduce sexually, which million years collide. Quartz-rich sandstones, shales, and makes genetic diversity possible, as well as the ago are deposited over the continents. ability to adapt to and survive environmental levels increase as life on Earth develops changes. Multi-celled, soft-bodied marine the ability to obtain energy through organisms (metazoans) evolve. . The late Proterozoic is an "Ice House" world.

Archean Eon The Earth's permanent crust is formed. Vast The earliest life forms evolve in the seas. They 3.9-2.5 billion amounts of metallic minerals are deposited. The are the prokaryotes—single-celled organisms years ago oceans and atmosphere result from volcanic with no nucleus—cyanobacteria (blue-green outgassing. algae). The earliest bacteria obtain energy through chemosynthesis (ingestion of organic molecules). Hadean Eon The Earth forms as a solid planet. No evidence of life yet known. (Azoic) 4.5-3.9 billion years ago